The present invention relates to an electron gun for a cathode-ray tube, and more specifically to an electron lens of an electron gun for focusing at least one electron beam, preferably two or more electron beams.
Conventionally, a cathode-ray tube includes at least one electron gun. The electron gun comprises a beam forming section for producing an electron beam and a main lens section for focusing the electron beam on the target. The spot diameter of the electron beam on the target is a very important factor to determine the performance of the cathode-ray tube. The spot diameter on the target should preferably be minimized, depending on the performance of the electron gun. Improvement of the performance of the main lens section is an effective measure for improving the performance of the electron gun.
The main lens section is chiefly composed of an electrostatic electron lens. In the electron lens region, a plurality of electrodes each having an aperture are coaxially arranged so as to be applied with predetermined voltages. There may be several, types of such electrostatic electron lenses which vary according to the variety of voltages. For higher performance of the main lens section, however, it is necessary to increase the size of the aperture, thereby increasing the lens aperture in the optical sense, or to lengthen the separation distance of the electrodes to cause a gradual potential change in the region around the electrodes, thereby forming a long-focus lens having a long focal length.
The prior art electron gun for a cathode-ray tube is sealed in a cylindrical glass tube, i.e., the neck portion of a cathode-ray tube. Therefore, the size of aperture of the electrodes or the lens diameter is practically restricted by the diameter of the cylindrical glass tube. Also, the separation distance of the electrodes is limited so that an electrostatic focusing field formed between the electrodes may not be influenced by any other undesired electric fields in the cylindrical glass tube. In a color picture tube, in particular, if a plurality of electron guns are arranged in line, narrower intervals between the electron guns will make it easier to converge a plurality of electron beams on the same point on the whole surface of a screen. In consideration of deflection, moreover, the narrow intervals between the electron guns improve the economy of electric power. The narrower intervals, however, require further reduction in size of the apertures of the electrodes.
In the cathode-ray tube as described above, the lens performance is expected to be improved by the use of a long-focus lens which can produce, without extension of the separation distance of the electrodes, an effect equivalent to that obtained with use of a longer separation distance. There are proposed several electrostatic electron lenses for such a cathode-ray tube, including a "distributed Einzel lens" disclosed in U.S. Pat. No. 3,895,253 by Schwartz et al., a "tripotential lens" disclosed in U.S. Pat. No. 3,995,194 by Blacker et al., a "multi-element bipotential lens" disclosed in U.S. Pat. No. 3,932,786 by Campbell, and a "single-element bipotential lens" disclosed in U.S. Pat. No. 4,124,810 by Bortfeld et al. Among these lenses, the distributed Einzel lens disclosed in U.S. Pat. No. 3,895,253 is not practical because, in this lens, electric discharge is liable to be caused between the relatively low voltage of the beam forming section and the higher anode voltage at the main lens section nearest thereto.
In the tripotential lens disclosed in U.S. Pat. No. 3,995,194 and the single-element bipotential lens disclosed in U.S. Pat. No. 4,124,810, three cylindrical electrodes with the same diameter are arranged along electron beams for low, middle, and high voltages, so that a gradual potential change is produced at the main lens section. An optimum lens performance may be obtained if the length of the middle-voltage electrode is substantially equal to the radius of the electrode aperture. However, the lens performance cannot further be improved.
For additional improvemnt of the lens performance, therefore, the multi-element bipotential lens disclosed in U.S. Pat. No. 3,932,786 is proposed. In an electron gun using this lens, however, resistors arranged near the individual electrodes are small. Thus, the electron gun of this type is unfit for practical use. Moreover, since the voltages of the electrodes are picked up at narrower intervals from the small resistor, the construction and manufacture of the electron gun are complicated. The small gaps between the electrodes facilitate the flow of leakage current between the electrodes. Consequently, undesired current is produced by the leakage current, beam impact hit on the electrodes, and other factors, resulting in a change of electrode potential and lowering the lens performance. These drawbacks make it very hard to put the electron gun of this type to practical use.
To increase the diameter of the electron lens, moreover, electron guns of the following types are conventionally proposed. In an electron gun for a color picture tube disclosed in Japanese Patent Application Disclosure No. 124933/80, three electron lenses are formed overlapping one another. In another electron gun stated in "Proceedings of the Third Interational Display Research Conference, Japan Display, 1983, pp. 268 through 271, apertures of electrodes are conical. In an electron gun disclosed in Japanese Patent Application Disclosure No. 103246/82, moreover, projections are formed around three apertures. In these electron guns, the diameter of each electron lens is increased so that the lens performance is improved in some measure. For further improved lens performance, the separation distance of the electrodes need to be increased. This separation distance cannot, however, be increased, since it is influenced by undesired electrostatic fields in the neck.